This research program aims to investigate photoreceptor-specific phosphoproteins which complex with the beta-gamma subunits of transducin (T beta gamma) and which may participate in light- modulated activities of rod photoreceptors. The 33K/T beta gamma complex of mammalian rods will be characterized in terms of its interaction with transducin, its phosphorylation dependent binding to ROS membranes and its light-modulated dephosphorylation. The 33K (33 kilodalton) protein which is phosphorylated in darkness and dephosphorylated during illumination forms a complex with the beta- gamma subunits of transducin. Immunocytochemistry using monospecific antibodies against bovine 33K shows that the 33K protein is restricted to rod outer and inner segments of bovine and mouse photoreceptors. Following centrifugation of retinal homogenates, the 33K protein partitions between the soluble and membrane-bound fractions. The soluble fraction contains predominantly unphosphorylated 33K/T beta gamma complex, whereas the membrane-bound 33K pool appears to be mainly phosphorylated. These findings suggest that the partitioning of 33K between the soluble and membrane fractions is associated with the state of 33K phosphorylation and this hypothesis will be evaluated. Additionally, the interaction of 33K/T beta gamma complex with transducin and possible modulation of PDE-activation cascade will be tested and the enzymes that phosphorylate or dephosphylate 33K will be characterized. A 44K protein of toad retinas which shares antigenic determinants with the 33K protein and which appears to complex with T beta gamma will be analyzed for its cyclic nucleotide-dependent phosphorylation, its light-modulated dephosphorylation and its selective localization. It is envisioned that there is a family of photoreceptor-specific phosphoproteins which complex with the beta-gamma subunits of transducin. Whereas, the molecular mass of the phosphoproteins may differ from that of mammalian rods in lower vertebrates, each phosphoprotein/T beta gamma complex probably serves a common function in photoreceptor activities.